This invention relates to nickel metallization in the preparation of semiconductor packaging devices, e.g. ceramic carriers employed in the packaging of semiconductor chips.
The increased performance and circuit/bit densities of today's semiconductor chips require corresponding technological advancements in chip packaging. Since the introduction of the leadless ceramic chip carrier, the chip carrier packaging concept has seen increasing use. Ceramic chip carriers typically make use of alumina-based substrates and have discrete areas of multi-layer metallization bonded to the ceramic substrate. These areas of metallization comprise in sequence (a) a base metallization layer of tungsten or molybdenum bonded to the ceramic substrate, (b) a layer of nickel bonded to the initial, or base, layer and (c) a layer of gold bonded to the nickel layer.
As a practical matter tungsten is the metal of choice for the base metallization although the use of molybdenum has been reported by one manufacturer for internal consumption.
In the typical fabrication of multi-layer ceramic substrates, alumina powder is mixed with glass frit and organic chemicals to form a slurry. This slurry is cast into sheets having a controlled thickness, which sheets are then blanked into various sizes and shapes, and via holes are punched. These green sheets are then screen printed with tungsten (or molybdenum) to form the base metallization. These metallized green sheets are stacked, and laminated together, followed by cofiring (i.e., sintering) in hydrogen or a hydrogen-nitrogen mixture with the heating schedule usually peaking at 1550.degree. C.-1650.degree. C. Thereafter, these sintered substrates are processed to apply nickel metallization over the exposed discrete areas of sintered tungsten or molybdenum. This is followed in turn by gold metallization of the nickel surfaces. Actual compositions of the slurry and specifics of the processing can be expected to vary from manufacturer to manufacturer.
The tungsten or molybdenum metallization is about 10 micrometers thick and is very porous. The nickel layer applied thereto is typically 2-5 micrometers thick, this layer being applied by electrolytic or electroless nickel plating. The thin layer of gold typically 1-2 micrometers thick is applied to accommodate die attachment, wire bonding and sealing. For good hermeticity (and for other reasons) it is important that the nickel and the gold metallizations contain as few pores as possible.
The sintered multi-layer ceramic bodies provided with the discrete areas of multi-layer metallization are subsequently subjected to brazing, chip joining and capping operations.
In existing ceramic chip carrier manufacture, nickel metallization is accomplished by the use of either an electrolytic or electroless plating process. A high purity nickel deposit generally can be obtained by electrolytic plating. It is well known, however, that this process has several major drawbacks including the following: (a) because of the need for an externally applied electrical current, it is often difficult to plate articles with complex shapes and circuitry; (b) for the same reason, the nickel coating is generally very nonuniform being thicker in well-exposed areas and substantially thinner at corners, and (c) the coating tends to be quite porous.
Because of these and other disadvantages of the electrolytic mode of plating, nickel metallization of ceramic chip carriers increasingly is carried out by electroless plating. This latter method can plate articles regardless of complexity of shape or circuitry with a relatively uniform coating thickness. Unlike electrolytically deposited nickel, however, electrolessly deposited nickel typically contains a high level (usually in excess of 1 wt. %) of boron or phosphorus. These impurities have been associated with the introduction of high internal stress in the coatings. U.S. Pat. No. 4,407,860--R. Fleming et al. discusses this problem and proposes a composition for the base metallization together with a post-coating treatment that purportedly provides stress-free pure nickel metallization.
Another concern in the use of electroless plating of nickel is the variability of the coating (e.g. thickness and completeness of coverage), which in large part is due to the extreme sensitivity of electroless nickel plating to the surface chemistry of the base metallization (tungsten or molybdenum). Still another concern is the limited selectivity of the coating, i.e., the coating often results in the deposition of nickel on the alumina substrate as well as on the tungsten. This latter problem, which is a source of electrical failure, can often be attributed to the presence of residue of a chemical, such as palladium chloride, used to activate the tungsten surface prior to applying the electroless nickel coating.
The over-all process of nickel metallization, whether by electrolytic or electroless plating, entails a great many steps. The reasons are basically as follows:
(a) a number of pre-cleaning steps (including acid and/or alkaline bath cleaning) are required, because of the sensitivity of the coating methods to the surface chemistry of tungsten, PA1 (b) in electroless plating several steps are required to activate the tungsten surface with a chemical, such as palladium chloride, and then to rinse off the excess chemical and condition the surface prior to nickel plating, PA1 (c) several more steps are required to promote adhesion of the nickel layer to the base metallization; this is often done by applying a first coating of nickel, heat-treating this coating at an elevated temperature to improve bonding to the tungsten, and then applying a second layer of nickel, and PA1 (d) a number of post-coating cleaning steps are required prior to gold plating to rinse off various chemical residues trapped in the pores and other defects in the nickel coatings.
It is, therefore, not uncommon to require 20-30 steps starting from the cofired chip carrier substrate to reach the stage at which the chip carrier is ready for application of the gold plating. It would be particularly advantageous to be able to eliminate such multiple-steps, which are cumbersome and costly. It would also be advantageous to reduce the incidence of pores in the nickel coatings and have them free of boron and phosphorus content.
By the process of nickel metallization of this invention, it is proposed to markedly reduce the processing time and cost of the nickel metallization, while simultaneously providing a superior reproducible nickel coating free of boron and phosphorus. Pack chromizing processes are known and used in industry primarily to produce chromium coatings to protect substrates, such as low carbon steel, against corrosion.